The present invention relates to a power transmission apparatus for a vehicle, and more particularly, to such a power transmission apparatus having an engine starting function which is compact in size and has a high degree of freedom and accuracy in controlling the transmission of engine torque.
Many power transmission apparatuses for vehicles have hitherto been proposed which have not only a speed-changing function but also a vehicle starting-off function. All of the conventional power transmission apparatuses have the following drawbacks. Specifically, in the case of a manually operated gear transmission with a driver-operated clutch for example, when starting off the vehicle or when the gears are changed, the clutch in the transmission is completely connected or engaged only after passing through a partially connected or engaged state which lasts for about one to five seconds. Specifically, during the gear changing operation, the driver operates the clutch pedal in synchronization with the manipulation of a shift lever for changing the gears so as to interrupt the operative connection between the output shaft of the engine and the input shaft of the transmission. In the course of such disconnection or disengagement, the driver manipulates the shift lever into a desired gear and thereafter again operates the clutch pedal so as to re-establish connection or engagement between the engine output shaft and the transmission input shaft. At this time, it is necessary for the driver to connect or engage the clutch for the purpose of preventing the generation of torque shock or torque fluctuations. Such clutch operation requires considerable skill on the part of the driver.
Moreover, if the driver is forced to repeatedly perform such clutch pedal operations, he or she may become fatigued making it difficult for the driver to perform proper clutch pedal operation at all times. As a result, the power transmission system requires extra or additional torque transmission capacity in order to avoid impairment or damage thereto due to great torque fluctuations.
Moreover, in manual transmissions where the driver has to manually operate the clutch pedal and the shift lever, there will be an attendant energy loss due to sliding friction between partially engaged clutch members.
Conversely, in the case of a hydraulic torque converter having a constantly engaged gear transmission hydraulically operated to change gears in an automatic manner, when the vehicle is started off from a standstill, engine torque is gradually transmitted between the input and output members of the torque converter through the intermediary of a fluid such as hydraulic oil. Thus, clutch operations are automatic and no manual operation on the part of the driver is necessary. Nonetheless, instead of the driver's clutch pedal operation, etc., a hydraulic circuit including a hydraulic pump is required so that the entire apparatus becomes large in size and complicated in construction, while energy loss from circulating the hydraulic fluid, sliding loss in the hydraulic converter and the like is great.
Moreover, with the above-described conventional apparatuses, a starter motor and a starter clutch for starting the engine are required independently of and in addition to the power transmission apparatus, thus reducing space efficiency.